In addition to using the standard types, users can derive their own types (called (userdefined types). User-defined types are between two and eight characters long and start with $ or SYS. A redirection mechanism allows user-defined types to be used instead of standard types in all commands and programs (see section "Library assignment")).
Supertype
Each user-defined type can be assigned to a supertype, i.e. a type above it in the type tree, with MODIFY-TYPE-ATTRIBUTES. LMS then processes the members of a user-defined type in the same way as it processes the members of its supertype. The supertype itself can, on the other hand, be a user-defined type. This may result in several type trees being created, which should each have a standard type at its peak.
Base type
The type at the peak of a type tree is called the base type and should be a standard type. It defines how the user types derived from it are to be handled by LMS.
Example
D has no supertype and is therefore its own base type.
D1 has D as its supertype and its base type.
D3 has D1 as its supertype and D as its base type.